As a method of separating and conveying stacked sheets, such as documents and recording sheets, a separating and conveying method using frictional force has been used.
The separating and conveying method using frictional force typically uses, for example, a rubber feeding roller, and as a result the frictional force changes over time due to abrasion and other factors, such that the conveying performance is degraded. Further, when sheets of non-uniform (i.e., varying) coefficient of friction or sheets having different coefficients of friction are separated and conveyed in the same separating and conveying operation, a feeding failure occurs in some cases, which includes simultaneous multiple feeding of a plurality of sheets and a failure to separate sheets. Further, in some cases, the sheets are sullied due to a configuration that separates the sheets by applying pressure thereto in the sheet conveying operation.
In view of the above, an electrostatic method as one type of non-frictional separation method has been proposed, which generates an electrical field in a dielectric belt and brings the dielectric belt into contact with a sheet to simultaneously attract the sheet and separate the sheet from other sheets. Such a technique is disclosed, for example, in Japanese Patent Application Publication No. JP-2003-237969-A1.
FIGS. 1A to 1C illustrate a background example of a sheet conveying device 220 according to JP-2003-237969-A1. FIG. 1A illustrates a standby state of the sheet conveying device 220. FIG. 1B illustrates a state in which a sheet is attracted to an attraction belt. FIG. 1C illustrates a state in which the sheet is conveyed.
The sheet conveying device 220 according to JP-2003-237969-A1 as illustrated in FIGS. 1A to 1C includes an attraction separation unit 201 to separate and convey an uppermost sheet S1 of a sheet stack S. The attraction separation unit 201 includes an attraction belt 200 that is stretched taut by a driven roller 202, a drive roller 203, and a tension roller 204. Further, the attraction separation unit 201 includes a charge roller 205 that serves as a charging device that charges a surface of the attraction belt 200, and a roller 206 that contacts the uppermost sheet S1 and rotates together with the uppermost sheet S1.
The drive roller 203, the driven roller 202, the tension roller 204, the charge roller 205, and the roller 206 are rotatably supported by a side plate of the attraction separation unit 201. The side plate is configured to be rotatable around a rotary shaft of the drive roller 203 that serves as an upstream extension roller in a sheet conveyance direction.
In conveyance of the uppermost sheet S1 of the sheet stack S stacked on a bottom plate 211 of a sheet container 210, the bottom plate 211 is lifted to bring the uppermost sheet S1 into contact with the roller 206. Then, the attraction belt 200 is rotated, and an alternating charge is applied to the surface thereof by the charging roller 205. Then, the attraction separation unit 201 is rotated in the counterclockwise direction in FIGS. 1A to 1C around the rotary shaft of the drive roller 203, so that an area of the attraction belt 200 located between and stretched by the driven roller 202 and the tension roller 204 (i.e., a sheet attraction surface) is brought into contact with the uppermost sheet S1 to electrostatically attract the uppermost sheet S1 to the attraction belt 200 (see FIG. 1B). Then, the attraction separation unit 201 is rotated in the clockwise direction to cause a sheet electrostatically attracted to the attraction belt 200 to move together with the attraction belt 200. In this state, the uppermost sheet S1 is wrapped around a portion of the attraction belt 200 in contact with the roller 206 as a fulcrum, and a restorative force acts on the sheet. The attractive force of the uppermost sheet S1 toward the attraction belt 200 is stronger than the restorative force of the uppermost sheet S1, and thus the uppermost sheet S1 moves together with the attraction belt 200.
By contrast, the distance between the subsequent sheet (i.e., the second sheet in the sheet stack S) and the attraction belt 200 is greater than the distance between the uppermost sheet S1 and the attraction belt 200 and the attractive force of the subsequent sheet toward the attraction belt 200 is weaker than the restorative force of the subsequent sheet, and therefore the subsequent sheet separates from the attraction belt 200 (see FIG. 1C). Then, the attraction belt 200 is rotated to convey only the uppermost sheet S1 attracted thereto toward a pair of conveying rollers.
In the sheet conveying device 220 disclosed in JP-2003-237969-A1, the center of swing of the attraction separation unit 201 is set to a position upstream in the sheet conveyance direction of the area of the attraction belt 200 coming into contact with the uppermost sheet S1 (i.e., the sheet attraction surface). By so doing, the attraction belt 200 can be separated from the sheet stack S simply by the swing of the attraction separation unit 201. Accordingly, there is no need to provide a device for lifting the attraction separation unit 201.
Further, with the roller 206 contacting a sheet, favorable separation performance can be obtained. Further, the roller 206 is configured to rotate together with a sheet, and does not rotate after the trailing edge of the uppermost sheet Si passes under the roller 206. Accordingly, the subsequent sheet does not receive the conveying force.
In the sheet conveying device 220 disclosed in JP-2003-237969-A1, however, the rotary shaft of the upstream tension roller (i.e., the drive roller 203) is set as the center of swing of the attraction separation unit 201. Therefore, to set the location of the center of swing of the attraction separation unit 201 to a position upstream from the sheet attraction surface in the sheet conveyance direction, three rollers (i.e., the driven roller 202, the drive roller 203, and the tension roller 204) are needed to keep the attraction belt 200 taut. Different from these rollers 202, 203, and 204, the roller 206 is also disposed to provide the restorative force to the sheet when separating the sheet. This configuration, therefore, increases the number of components and therefore also the cost of the sheet conveying device 220.